1. Field of the Invention
The present invention relates generally to enlarging the diameter of a subterranean borehole and, more specifically, to enlarging the borehole below a portion thereof which remains at a lesser diameter. The method and apparatus of the present invention effects such enlargement using a stability-enhanced bi-center bit.
2. State of the Art
It is known to employ both eccentric and bi-center bits to enlarge a borehole below a tight or undersized portion thereof.
An eccentric bit includes a pilot section, above which (as the bit is oriented in the borehole) lies an eccentrically laterally extended or enlarged cutting portion which, when the bit is rotated about its axis, produces an enlarged borehole. An example of an eccentric bit is disclosed in U.S. Pat. No. 4,635,738.
A bi-center bit assembly employs two longitudinally-superimposed bit sections with laterally offset axes. The first axis is the center of the pass-through diameter, that is, the diameter of the smallest borehole the bit will pass through. This axis may be referred to as the pass-through axis. The second axis is the axis of the hole cut as the bit is rotated. This axis may be referred to as the drilling axis. There is usually a first, lower and smaller diameter pilot bit section employed to commence the drilling and establish the drilling axis. Rotation of the bit remains centered about the drilling axis as the second, upper and larger radius main, or reamer, bit section extending beyond the pilot bit section diameter to one side of the bit engages the formation to enlarge the borehole. The rotational axis of the bit assembly then rapidly transitions from the pass-through axis to the drilling axis when the full diameter or "gage" borehole is drilled.
Rather than employing a one-piece drilling structure, such as an eccentric bit or a bi-center bit, to enlarge a borehole below a constricted or reduced-diameter segment, it is known to employ an extended bottomhole assembly (extended bi-center assembly) with a pilot bit at the distal end thereof and a reamer assembly some distance above. This arrangement permits the use of any standard bit type, be it a rock bit or a drag bit, as the pilot bit, and the extended nature of the assembly permits greater string flexibility when passing through tight spots in the borehole as well as the opportunity to effectively stabilize the pilot bit so that the pilot hole and the following reamer will take the path intended for the borehole. The assignee of the present invention has designed as reaming structures so-called "reamer wings" which generally comprise a tubular body having a fishing neck with a threaded connection at the top thereof and a tong die surface at the bottom thereof, also with a threaded connection. The upper mid-portion of the reamer wing includes one or more longitudinally-extending blades projecting generally radially outwardly from the tubular body, the outer edges of the blades carrying superabrasive (also termed "superhard") cutting elements, commonly termed PDC's (for Polycrystailine Diamond Compacts). The lower mid-portion of the reamer wing may include a stabilizing pad having an arcuate exterior surface the same or slightly smaller than the radius of the pilot hole on the exterior of the tubular body and longitudinally below the blades. The stabilizer pad is characteristically placed on the opposite side of the body with respect to the reamer wing blades so that the reamer wing will ride on the pad due to the resultant force vector generated by the cutting of the blade or blades as the enlarged borehole is cut. U.S. Pat. No. 5,497,842, assigned to the assignee of the present invention and incorporated herein for all purposes by this reference, is exemplary of such reamer wing designs. U.S. Pat. No. 5,765,653, also assigned to the assignee of the present invention and incorporated herein for all purposes by this reference, discloses and claims more recent improvements in reamer wings and bottomhole assemblies for use therewith, particularly with regard to stabilizing reamer wings and bottomhole assemblies.
As one might suspect from the foregoing descriptions of their respective structures, bi-center bits are more compact, easier to handle for a given hole size, more suitable for directional drilling bottomhole assemblies (particularly those drilling so-called "short" and "medium" radius nonlinear borehole sections), and also less expensive to fabricate than reamer wing assemblies. However, stability of bi-center drill bits remains a significant, recognized problem.
For example, an Oil & Gas Journal article entitled "Use of bi-center PDC bit reduces drilling cost," Nov. 13, 1995, pp. 92-96, notes that the bi-center bit is impossible to "stabilize fully because the largest stabilizer size that can be used is the pass-through diameter, not the hole diameter". Further, the article notes that the bi-center bit is an unstable design due to the high loading on the pilot bit cutters opposite the reaming cutters (those on the main or reamer bit section), which are all located on one side of the hole. The result of these inadequacies is demonstrated (as noted in the aforementioned article) by an unacceptably severe tendency of these prior art bi-center bits to drill off their intended paths, or "walk," in a particular direction, resulting in a "dogleg" in the borehole, particularly undesirable in high precision, state-of-the-art directional and navigational well drilling.
Prior art bi-center bits, due to the above-noted imbalanced loading, also tend to exhibit the well-recognized phenomenon of bit "whirl," wherein a drill bit rotates or "whirls" about a center point offset from the geometric center of the bit in such a manner that the bit tends to precess or rotate backwards (opposite the direction of drill string rotation) about the borehole. One approach to alleviate bit whirl in conventional bits is to attempt to perfectly balance the radial and tangential cutter forces to achieve a laterally-balanced bit, as disclosed in U.S. Pat. No. 4,815,342. This approach will obviously not work with a bi-center bit due to the overwhelming dominance of the imbalanced side forces generated by the reamer bit section. Another approach, disclosed in U.S. Pat. No. 5,010,789, has been to intentionally imbalance the radial and tangential cutter forces of a conventional bit to direct a resultant force vector to one side of the bit, which side includes a bearing surface pushed by the force vector into substantially constant contact with the sidewall of the borehole. A variation of this approach has been used to stabilize reamer wing bottomhole assemblies, as disclosed in the above-referenced, commonly-assigned '842 and '653 patent, wherein a discrete stabilizer pad has been placed immediately below and opposite the blades of the reamer wing. However, the longitudinally compact configuration of bi-center bits also renders the discrete stabilizer pad approach unworkable, there being no location on the bit suitable for placement of such a structure.
The inventors herein have reflected at length on the instability problems of bi-center bits, and concluded that the aforementioned loading problem is not strictly the result of the placement of cutters on the reamer bit section, but of the relative, drastically misaligned orientations and difference in relative magnitudes of the composite or resultant radial force vector generated by the group of cutters on the pilot bit section in comparison to the radial force vector generated by the group of cutters on the reamer bit section. Such misalignment causes the bi-center bit to tilt or cock in the borehole, as the longitudinally offset, radially misaligned force vectors augment each other, driving the bit away from a desirable orientation wherein the longitudinal axis of the bit and that of the borehole are coincident, or, at the least, mutually parallel with an extremely small lateral offset. To further explain the problem, reference is made to FIG. 1 of the drawings, wherein an exemplary prior art bi-center bit 10 is schematically depicted in borehole B. The resultant radial force vector F.sub.1 of the pilot bit section 12 is directed to the right of the page, while the longitudinally-offset resultant radial force vector F.sub.2 of the reamer bit section 14 is directed to the left of the page, the two force vectors thus tending to cock or tilt the bit about a horizontal axis of rotation A lying between the pilot bit and reamer bit sections. The relatively large, highly directional resultant force vector F.sub.2 generated by the reamer bit section cutters also contributes to instability problems in prior art bi-center bits, as such bits employ gages 16 having inadequate surface area radially opposing force vector F.sub.2 to maintain the pilot bit section 12 in a stable position concentric with an ideal longitudinal axis L of the borehole, and thus the bi-center bit tends to drill an oversize and out-of-round pilot borehole PB which the reamer bit section follows, drilling an undersized reamed hole.
Existence of the above-mentioned dominant force vector F.sub.2 has been previously recognized, and solutions to bi-center bit imbalance proposed, in SPE/IADC Paper No. 29396, "New Bi-Center Technology Proves Effective in Slim Hole Horizontal Well". However, one part of the proposed solution involved developing a greater imbalance in the lateral force vector F.sub.1 of the pilot bit and to direct it in opposition to that of vector F.sub.2, as shown in FIG. 1. As noted above, the inventors herein have recognized that such radially opposed forces actually exacerbate the imbalance problem and promote tilting or cocking of the bit in the borehole.
Thus, given the noted deficiencies of prior art attempts to reduce bi-center bit imbalance, there remains a need for a bi-center bit affording a high degree of stability, so that the otherwise advantageous characteristics of this type of bit design may be fully utilized.